1. Technical Field
The present embodiment relates generally to a hybrid cord and a high-performance radial tire including the same and, more particularly, to a hybrid cord having different physical properties on a stress-strain curve, excellent process-related manufacturing efficiency, and excellent quality uniformity compared to a conventional hybrid cord, and a high-performance radial tire including the same.
2. Description of the Related Art
With the improvement of the performance of vehicles and the betterment of road conditions, driving speed has gradually increased. In line with this tendency, research into tire cords used as the rubber reinforcing materials of tires have been actively carried out in order to enable the stability and durability of tires to be maintained during high-speed driving.
Since some materials used in tire cords have high strength but have low tensile elongation at their breaking points, they are not suitable for applications that require high tensile elongation at their breaking points. In contrast, since some materials have very high tensile elongation at their breaking points but do not have sufficient strength, they cannot be used for some applications that require high breaking pressures or the bearing of heavy loads.
To overcome these problems, a hybrid cord formed by combining and twisting together fiber having high fatigue resistance and a high tensile elongation at a breaking point and fiber having high strength was developed. The hybrid cord in the form of plied yarn is manufactured by twisting together two threads based on material having a small initial modulus (for example, polyamide 66) and material having a large initial modulus (for example, aramid) so as to assign tensile modulus that decreases in the case of small deformation and increases in the case of large deformation.
The hybrid cord having a plied yarn structure is disadvantageous in that speed decreases in the case of small deformation, and driving noise is caused by premature “stiffening,” thereby degrading riding comfort based on a tire.
To overcome these problems, Korean Patent Application Publication No. 2004-0077875 discloses a technology in which the ratio of the final tangent modulus to initial tangent modulus of a hybrid cord is set to a value equal to or greater than 10, thereby reducing driving noise. In the above-referenced patent publication, polyamide 66 yarn is used as core yarn, and aramid yarn is used as covering yarn. In this case, to form polyamide into core yarn and aramid into covering yarn, it is necessary to assign a larger Z twist to the aramid than to the polyamide or to twist the polyamide in a direction opposite the direction in which the aramid is twisted. This method has a disadvantage in that it cannot be implemented by a state-of-the-art direct cabler, and suffers from significant losses in processing efficiency, such as an increase in the manufacturing period, because an old-fashioned ring twister must be used to perform the method. When a low tension is applied to dipping and heat treatment, a core-covering structure constructed during the plied yarn formation process can be maintained, and an aramid-polyamide 66 hybrid cord obtained as described above can be configured such that the ratio of a cutting modulus to an initial modulus is equal to or greater than 10. The reason why such a large modulus ratio is applied is to assign a low modulus to the initial section of a stress-strain curve in order to facilitate manufacturing and assign a high modulus to the final section of the stress-strain curve in order to improve the capability to resist deformation in a tensile direction in a finished tire.
FIG. 1 shows the stress-strain curve of an aramid-polyamide 66 hybrid cord manufactured using a conventional method. In FIG. 1, an initial modulus Einitial is 14 gf/d, and a final modulus Eterminal is 257 gf/d. Accordingly, the ratio between the final modulus and the initial modulus is 18.3, which is larger than 10, as described above. However, since the material of a fiber cord, such as a textile cord, is not in a solid state but corresponds to a thin fiber assembly, significant error may occur during measurement, and thus it is insufficient to simply define physical properties using the ratio between a final modulus and an initial modulus.
Furthermore, an excessively low initial modulus in the aramid-polyamide 66 hybrid cord may bring about the disadvantage of causing differences among products in subsequent processes including a rolling process. When tension is artificially reduced during a process of the heat treatment of a cord in order to achieve a low initial modulus, a problem arises in that differences among the physical properties of cords increase. When the initial modulus of the initial section of a stress-strain curve is excessively reduced in order to facilitate manufacturing, it is excessively lower than that of process conditions, and thus a problem may arise in that a cord may be easily deformed when tensile force is applied to the cord in the axial direction thereof during processing, with the result that the stability of dimensions may be degraded during a manufacturing process. Furthermore, a disadvantage arises in that differences among the physical properties of cords occurring during cord manufacturing processes may influence the uniformity and dimensions of finished tires.
In the conventional aramid-polyamide 66 hybrid cord, an initial modulus is set to a very low value in order to increase the ratio of a final modulus to the initial modulus. Accordingly, various problems occur in terms of the manufacture of a cord, resulting in an increase in manufacturing costs. For example, when a structure in which polyamide 66 yarn is disposed in a core and is then covered with aramid yarn is constructed, an aramid-polyamide 66 cord having a core-covering structure is generally manufactured using a method in which a twisting machine applies different twists to respective plies and tension is scarcely applied during a heat treatment process that is performed after dipping, thereby inducing the self-contraction of the polyamide 66 yarn. However, since all processes must be performed without tension, a disadvantage arises in that a very slow processing speed must be maintained, and differences among physical properties, such as the intermediate elongations of respective cords, etc., increase. Furthermore, the very low initial modulus makes it difficult to maintain uniform physical properties during the rolling and cutting/forming processes of a tire manufacturing process in which tensile force is applied, and ultimately influences the uniformity of the physical properties of tires.